Traditionally, such a signaling magnetic plug is mounted on a casing containing rotary parts, such as gearwheels or bearings, that are immersed in said liquid.
In known manner, the function of the liquid circuit is generally to lubricate and/or cool the rotary parts.
It happens that the rotary parts suffer wear during their lifetime, which wear may be normal, e.g. resulting from contact friction between two gearwheels, or else it may be completely abnormal, e.g. as a result of impacts or intense friction between rotary parts due to intense and abnormal vibration propagating in the casing, and resulting in particular from a rotary part breaking. Degradation of an aircraft engine can also give rise to abnormal wear of component parts of the engine.
Whatever the cause, the wear of such parts leads to particles forming that detach from the part and that are entrained by the liquid in the liquid circuit.
Insofar as the rotary parts are generally made of metal, the particles that result from the parts wearing are electrically conductive and they are generally in the form of filings. Furthermore, the parts are usually made of a metal of the ferromagnetic type, such as iron, i.e. metal that is suitable for being attracted by a magnetic element such as a magnet.
In known manner, a signaling magnetic plug serves to detect the presence of such metal particles.
To do this, the first and second magnetic electrodes attract the metal particles while the liquid is flowing, and as a result particles accumulate on one and/or the other of the magnetic electrodes and tend to form a conductive bridge interconnecting the magnetic electrodes.
Once such a conductive bridge has formed, an associated electric circuit is in a position to detect that the conductive bridge has formed, thereby leading to a signal being sent to an operator, e.g. the pilot of the aircraft.
Such a magnetic plug is placed so as to detect the presence of particles, and on receiving a signal coming from the magnetic plug, which signal can be referred to as a “plug-on” signal, the operator must comply with utilization restrictions and perform a maintenance operation in order to determine the cause for the presence of the metal particles. For example, if particles are detected in the lubricating circuit of a helicopter engine, the “plug-on” signal requires the pilot to perform a safety landing as soon as possible for a single-engined machine, or in a multi-engine application to cause the engine concerned to idle. This affects the mission and the helicopter will need to be taken out of service for quite a long time for a maintenance operation.
Furthermore, it is often found that a magnetic plug delivers a “plug-on” signal for reasons other than abnormal wear of the engine (such signals are said to be “false-positives”).
This applies for example during the first two hundred hours of operation of the engine, in which period it is found that metal particles accumulate on the plug even though the rotary parts are not worn abnormally. The cause of the metal particles being present at the beginning of the lifetime of the engine is generally due to a running-in effect or to incomplete cleaning of the engine parts during manufacture of the engine. Metal particles can therefore be present in the oil circuit in the absence of abnormal wear.
This can also happen after the running-in period, where in normal operation the particles that result from normal wear of the engine accumulate on the magnetic plug and end up producing the “plug-on” signal.
Although such “plug-on” signals are not caused by abnormal operation of the engine, they are handled as though they were so caused, leading to the same consequences in terms of operations and maintenance.
Traditionally, one solution for mitigating such false-positives and for avoiding taking the helicopter out of service, is to burn away the particles that have accumulated on the plug.
A drawback is that information is lost concerning the reason for the magnetic plug generating the “plug-on” signal. In other words, the method consisting in burning off the particles makes it impossible during future a maintenance operation to verify whether the presence of the particles on the plug was due to earlier pollution or indeed to abnormal wear, and makes it impossible to identify the parts concerned by analyzing the material.
Particle counter systems are also known that serve to monitor variation in the size and number of particles, however such systems present numerous drawbacks, in particular their costs and their weight. They also present the drawback of containing electronic circuits, and it is then necessary to ensure that they are operating properly under operational conditions.